I'm not sure from the image how the pie was done, but I do know you can put things in the clay that will dissolve out after baking. If you put granulated sugar or salt in the colored clay, rolled it out into desired thickness of sheets, cut the desired shapes and baked it, then dissolve the granuals out of it, you might get close! Or maybe you need to do the colored sheets, cut the shapes and tap the edges into salt... you may get a better result. Bake the colored pieces then smash them inbetween the soft white clay to get the ooozzzing frosting. Also, you can run the clay through a cookie press type thing to get the frosting dolop on top.
As far as the stiff clay problem goes, I've been there.I love clay and all sculpting materials as far as that goes! OLD clay, COLD clay is stiff. The most important thing is "work softening" the raw clay. I played with clay for years before I bought my first pasta machine. The best $25.00 I ever spent! You can run it through the machine several times and "work warm" it easily. With out a pasta machine, you have to work it in your hands for a long time to soften really old or stiff clay. My elbows are weak and this hand warming process is hard for me. Sometimes I put it in a plastic bag in my pocket ... to warm it before I even pinch it to start with. Sometimes I take the signal from the clay and realize this piece is over the edge! So I slice it thin, roll it out and fold it over several times, mix a softer and pleasing color to it and end up with some really cool stone like patterns. These sheets won't ever make fluffy frosting but they're cool on their own.
There are also clay softners. Use with caution, I usually just make a mess!
